vii CHURCH OF THE THEOTOKOS PAMMAKARISTOS 155
vault, and overlooks the church by a small window pierced in the west cross arm. Each of the side bays is covered by a drum dome of sixteen concave bays pierced with eight windows and externally octagonal. The plaster has fallen away from these bays, allowing us to see that they are built in regular courses of brick with thick mortar joints and without any special strengthening at the lines of juncture or ribs between the compartments. Such domes, therefore, are not strictly ribbed domes but rather domes in compartments. The c ribs' no doubt do, by their extra thickness, add to the strength of the vault, but here, as in most Byzantine domes, their purpose is primarily ornamental.
The exterior of the chapel, like the fa9ade of S. Theodore (p. 247)3 presents a carefully considered scheme of decoration, characteristic of the later Byzantine school both here and in the later schools outside Constantinople. The southern wall is divided externally as it is also internally, into three stories, and forms two main compartments corresponding to the narthex and to the cross arm. They are marked by high arches of two orders, which enclose two triple windows in the upper story of the narthex and of the cross arm, The clue to the composition is given by the middle story, which contains the two large triple windows of the narthex and of the cross arm, and the two single lights of the angle compartment, one on each side of the cross arm triple light. These windows are enclosed in brick arches of two orders and linked together by semicircular arched niches, of which those flanking the narthex window are slightly larger than the rest, thus giving a continuous arcade of a very pleasant rhythmic quality.
In the lower story the piers of the arches round the triple windows are alone carried down through the inscribed string-course which separates the stories and forms the window-sill. The system of niches is repeated, flat niches being substituted for the angle compartment windows above.
The highest story contains the large single windows which light the cross arm and the gynecaeum, the former flanked by two semicircular niches, the latter by two